1. Field of the Invention
The present invention relates to a signal transmitter and a signal quality monitoring device which are preferably incorporated into an apparatus that monitors signal quality of a received signal and notifies an alarm representing the monitored signal quality to a external apparatus or an apparatus that uses such a alarm as information to switch communication lines (as a switching trigger) More specifically, the present invention relates to a signal quality device that is used for a synchronous network, such as SDH (Synchronous Digital Hierarchy) and SONET (Synchronous Optical NETwork defined by Bellcore and spread in North America).
2. Description of the Related Art
FIG. 6 of the accompanying drawings schematically shows an example of a synchronous network (ring network) in conformity with SDH and SONET. Synchronous network 100 is formed by a plurality of signal transmitters (nodes) A, B, C and D connected in a ring through optical paths (optical fibers) 103. Each of nodes A, B, C and D includes a higher-group apparatus 101 and a lower-group apparatus 102, which are mainly communicably connected by an electric line (e.g., a coaxial cable) 104.
Higher-group apparatus 101 byte-multiplexes a lower-group signal received from lower-group apparatus 102 to create higher-group signals suitable to be transmitted over ring network 100 (among nodes A, B, C and D), and then sent the created higher-group signals out to ring network 100. On the other hand, higher-group apparatus 101 demultiplexes lower-group signals that have been multiplexed into higher-group signals and sends the demultiplexed lower-group signals to lower-group apparatus 102. Lower-group apparatus 102 sends higher-group apparatus 101 lower-group signals that are communicated over a nonillustrated lower-group-side network, and sends the lower-group-side network lower-group signals that have been received from higher-group apparatus 101.
As a result, it is possible to add (lower-group) signals already served over the lower-group network to a higher-group signal communicated over ring network 100 in higher-group apparatus 101, and to drop lower-group signal that is to be sent to lower-group apparatus 102 and to be communicated over the lower-group network from a higher-group signal at higher-group apparatus 101.
In technology of SONET, assuming that a lower-group signal is OC-12 frame (Optical carrier at level 12; an optical transmission frame of approximately 622 Mb/s), a higher-group signal is OC-48 frame (an optical transmission frame of approximately 2.4 Gb/s) in which four OC-12 frames are multiplexed; and assuming that a lower-group signal is OC-48 frame, a higher-group signal is OC-192 frame (an optical transmission frame of approximately 10 Gb/s) in which four OC-48 frames are multiplexed.
Ring network 100 in conformity with SDH and SONET applies technology of APS (Automatic Path Switch) in which signal transmission paths are automatically switched based on a signal quality of a signal transmitted through optical path 103.
For example, higher-group apparatus 101 of one node of the four nodes in FIG. 6 (e.g., node A) transmits two signals identical in contents in two directions of WEST and EAST, respectively. If the transmission signals deteriorate over time and optical path 103 has a problem, higher-group apparatus 101 on the receiver-side node D obtains a quality of each of the transmission signals (a grade of an alarm in accordance with the quality) and selects one of the two transmission signals which is of better quality.
In order to realize the above-mentioned performance, each of nodes A, B, C, and D included in ring network 100 has a function to monitor a quality of a received signal by issuing an alarm and a function for APS. Assuming that an alarm that is issued in accordance with low quality of a signal takes two different grades (importance) each of nodes A, B, C, and D comprises an EAST-side receiving section 111E as a signal monitoring device 200, which includes alarm producing/canceling sections 111a and 111b each for each of the two grade of low quality and a PC (personal computer) 111c, and a WEST-side receiving section 111W identical in configuration with EAST-side receiving section 111E (signal quality monitoring device 200). In addition to the two receiving sections 111E and 111W, each nodes comprises an EAST-side transmitting section 112E including an APS section 113, a WEST-side transmitting section 112W including an APS section 114, and another APS section 115, as shown in FIG. 7 of the accompanying drawings. Thicker lines and thinner lines in FIG. 7 respectively represent communication lines for transmission signals transmitted over ring network 100 and communication lines for alarms and control signals.
In receiving section 111E (signal quality monitoring device 200), alarm producing/canceling circuit 111a discriminates whether a received signal (signal input (1)) is in a corresponding grade of low quality and, when the received signal stays in the corresponding grade for a predetermined producing time period, produces continuous output of an alarm corresponding to the grade of low quality. In the meanwhile, when it is discriminated that the received signal stays off the grade of low quality for a predetermined canceling time period, alarm producing/canceling circuit 111a cancels the continuous output of the alarm.
In the same manner, the other alarm producing/canceling circuit 111b discriminates whether one and the same received signal (signal input (1)) is in a grade of low quality higher than the grade that are discriminated in alarm producing/canceling circuit 111a and, when the received signal stays in the higher grade for another predetermined producing time period, produces continuous output of a higher-grade alarm corresponding to the higher grade of low quality. In the meanwhile, when it is discriminated that the received signal stays off the higher grade of low quality for a predetermined canceling time period, alarm producing/canceling circuit 111b cancels the continuous output of the higher-grade alarm. The lower- and higher-grade alarms will be described in detail later, and coincident outputs of the lower- and higher-grade alarms are not produced.
PC 111c receives results of producing/canceling continuous output of the lower- and the higher-grade alarms from alarm producing/canceling circuits 111a and 111b respectively, and then notifies the received results with respect to states of the alarms to a terminal (not shown in drawings) for system maintenance and provides each of APS sections 113, 114, and 115 with the states of the alarms as APS triggers. Receiving section 111W performs discrimination, and producing/canceling of output of alarms and then provides APS sections 113, 114, and 115 with APS triggers in the same manner performed by receiving section 111E.
Each of APS sections (a receiving signal selector) 113, 114 and 115 receives two signals, which are identical in contents and which are received via EAST direction and WEST direction (i.e., a signal input (1) and a signal input (2)), respectively, and selects (switches) either one of the signal inputs (1) and (2) based on a APS trigger received from PC 111c of receiving section 111E or 111W. One of the two signal input (1) and (2) is previously defied as a xe2x80x9cworkxe2x80x9d and the other as a xe2x80x9cprotectionxe2x80x9d. If the both signal inputs (1) and (2) stay off the corresponding grades of low quality thereby no alarm is output, APS sections 113, 114, and 115 select a signal input defined as the xe2x80x9cworkxe2x80x9d (in other words, a switching operation to the xe2x80x9cworkxe2x80x9d signal input is carried out). This switching operation is realized by software executed by a CPU.
Hereinafter, operations performed in each of nodes A through D having the above-mentioned configuration will now be described, assuming the signal input (1) (hereinafter also called signal (1)) and the signal input (2) (hereinafter also called signal (2)) as a work and a protection, respectively.
First of all, the signal (1) input into receiving section 111E is further input into APS sections 113, 114, and 115, and the signal (2) input into receiving section 111W is also input into APS sections 113, 114, and 115.
Each of APS sections 113, 114, and 115 switches the signals (1) and (2). Output from APS section 113 becomes a signal output (1); output from APS section 114 becomes a signal output (2); and output from APS section 115 is destined for lower-group apparatus 102.
In signal quality monitoring device 200 in receiving section 111E, alarm producing/canceling circuits 111a and 111b respectively discriminates whether the signal (1) is in the corresponding grade of low quality, and the results (signal quality information) of the discriminating are notified to PC 111c. PC 111c further notifies the signal quality information to a maintenance terminal, and to APS sections 113, 114, and 115 as APS triggers.
Signal quality monitoring device 200 in receiving section 111W also notifies signal quality information to a maintenance terminal, and to APS sections 113, 114, and 115 as APS triggers, likewise the receiving section 111E.
At that time, when alarm producing/canceling circuit 111a in receiving section 111E discriminates that the signal (1) is in the lower grade of low quality, PC 111c provides APS sections 113, 114, and 115 with a switching trigger, respectively. Upon receipt of the switching trigger, each of APS sections 113, 114, and 115 controls a switching operation performed therein so as to select the signal (2).
As mentioned above, in each of nodes A, B, C, and D included in synchronous network 100, APS triggers make APS sections 113, 114, and 115 select an output signal from two signal inputs (1) and (2) based on signal quality information thereby guaranteeing selection for an optical path through which signal of better quality is transmitted.
In a general practice, SONET defines the kinds of alarms that are to be produced/canceled in accordance with grade of low quality as the below table 1 when OC-48 frame is used as a main signal that is to be received by a signal transmitter and Sonet also defines producing time periods and canceling time periods corresponding to the individual alarms.
A producing time period represents a length of time period that causes continuous output of an alarm when a received signal staying in a grade of low quality corresponding to the output alarm for the producing time period; and a canceling time period represents a length of time period that causes cancellation of the continuous output of the alarm during the continuous output of the alarm when the received signal stays off the grade for the canceling time period.
Namely, output of an alarm is not produced or canceled immediately after discrimination whether a received signal is in or is out of the corresponding grade of low quality, however is produced or canceled after the received signal stays in or stay off the grade for predetermined producing period or canceling period (protection time period), respectively. As an advantage, it is possible to prevent frequent switching between producing and canceling continuous an alarm in a short time period due to shifting of grades of low quality of a received signal from occurring.
BER-SD(xe2x88x926) in table 1 is an abbreviation for Bit Error Rate Signal Degrade level at 10Exe2x88x926 that represents the occurrence of one-bit error in 10E6 bits in a received signal; and BER-SF(xe2x88x923) is an abbreviation for Bit Error Rate Signal Fail level at 10Exe2x88x923 that represents the occurrence of one-bit error in 10E3 bits of a received signal.
From the fact mentioned above, continuous output of BER-SD(xe2x88x926) alarm is produced when a received signal stays in BER-SD(xe2x88x926) of low quality for producing time period (787.5 ms), and the continuous output of BER-SD(xe2x88x926) alarm is canceled when a state of BER-SD(xe2x88x926) is restored and the received signal stays off the BER-SD(xe2x88x926) for the canceling time period (78.75 ms). In the same manner, continuous output of BER-SF(xe2x88x923) alarm is produced when a received signal stays in BER-SF(xe2x88x923) of low quality for producing time period (7.25 ms), and the continuous output of BER-SF(xe2x88x923) alarm is canceled when a state of BER-SF(xe2x88x923) is restored and the received signal stays off the BER-SF(xe2x88x923) for the canceling time period (725 ms).
Continuous output of LOF alarm is produced when a received signal (a SONET transmission frame) is in LOF (Loss Of Frame) or OOF (Out Of Frame) for the producing time period (in which it is discriminated that the received signal stays in OOF for 3 ms) in the table; and the continuous output of LOF alarm is canceled when the state of LOF is restored and the received signal stays off the LOF for the corresponding canceling time period (250 xcexcs) during the continuous output of LOF alarm. The producing time period for OOF is 625 xcexcs.
Continuous output of LOS (Loss Of Signal) alarm is produced when signal input is not received for the producing time period (12.5 xcexcs); and the continuous output of LOS alarm is canceled when the signal input is restored to be received and the state of receiving the signal input keeps for the canceling time period in the table during the continuous output of LOS alarm. Among the four kinds of alarms appear in table 1, SONET defines BER-SD(xe2x88x926) alarm as the lowest and LOS alarm as the highest grades.
A grade of low quality of a received signal can shift among the above-mentioned four grades, however coincident outputs of two or more alarms of different grades cannot be produced. Generally, a higher-grade alarm has a shorter producing time period and a longer canceling time period.
FIG. 8 of the accompanying drawings schematically shows a node in which a function for producing/canceling continuous outputs of the four alarms with different grades is incorporated in receiving section 111E (111W). Namely, receiving section 111E (111W) of each of the nodes A through D comprises a signal quality monitoring device 201, which includes BER-SD(xe2x88x926)-alarm producing/canceling section 111-1, BER-SF(xe2x88x923)-alarm producing/canceling section 111-2, LOF-alarm producing/canceling section 111-3, LOS-alarm producing/canceling section 111-4 and PC 111-5. Outputs (signal quality information) from each of alarm producing/canceling sections 111-1 through 111-4 causes PC 111-5 to provide APS sections 113, 114, and 115 with APS triggers, thereby switching optical paths. The other elements and other operations performed in each node are identical with those described with reference to FIG. 7.
In a general practice, transmission capabilities of each optical path 103 is not always constant and changes due to various factors, such as deterioration over time and a problem with the optical path 103. For this reason, results of discrimination with respect to a grade of low quality of a received signal, which discrimination is performed in a receiver-side node (signal quality monitoring device 200 or 201), is not always constant.
For example, when a grade of low quality of a received signal shifts from one grade to the immediate higher grade, the received signal may stay in the immediate higher grade for a relatively long time period, or may frequently shift between the one grade and the immediate higher grade at short-time intervals. In particular, when a sender-side node deteriorates over time, it takes a relatively long time to shift one to another grades of low quality whereupon such frequent shifting among respective different grades inclined to occur.
At that time, signal quality monitoring device 200 or 201 are forced to frequent producing and canceling of continuous outputs of alarms of different grades of low quality by the corresponding alarm producing/canceling sections in a short time period. Therefore, signal quality monitoring device 200 or 201 may output no alarm despite a received signal being in a grade of low quality (hereinafter this disagreement state a xe2x80x9cno-alarm statexe2x80x9d) because of the lengths of producing and canceling time periods set for each grade of low quality.
The occurrence of a no-alarm state will be described with reference to time chart FIG. 9, which illustrates operation carries out in signal quality monitoring device 200 of FIG. 7 (i.e. a received signal takes two grades of low quality).
No alarm states occurs at the time points (i) and (ii) if a grade of low quality of a received signal shifts between the different grades on respective Conditions A and B.
Condition A: the canceling time period for the higher-grade alarm is shorter than the producing time period for the lower-grade alarm;
Condition B: the producing time period for the higher-grade alarm is longer than the canceling time period for the lower-grade alarm.
The producing time period for the lower-grade alarm and the canceling time period for the higher-grade alarm are assumed as Condition A to be 4 ms and 3 ms respectively; and the producing time period for the higher-grade alarm and the canceling time period for the lower-grade alarm are assumed as Condition B to be 2 ms and 1 ms. Under the conditions (especially Condition A), when a grade of low quality of a received signal shifts from the higher grade to the lower grade (at T2 and T4), alarm producing/canceling circuit 111b has already canceled continuous output of the higher-grade alarm (at T21 and T41) due to expiration of the canceling time period (3 ms) until alarm producing/canceling circuit 111a produces continuous output of the lower-grade alarm (at T22 or T42) due to the received signal staying in the lower grade for the producing time period (4 ms) in conformity with Condition A. As a result, no-alarm states occur during time period (i) (T21 through T22 and T41 through T42).
On the other hand, when a grade of low quality of a received signal shifts from the lower grade to the higher grade (at T1 and T3), alarm producing/canceling circuit 111a has already canceled continuous output of the lower-grade alarm (at T11 and T31) due to expiration of the canceling time period (1 ms) until alarm producing/canceling circuit 111b produces continuous output of the higher-grade alarm (at T2 or T4) due to the received signal staying in the higher grade for the producing time period (2 ms) in conformity with Condition B. As a result, no-alarm states occur during time period (ii) (T11 through T2 and T31 through T4).
No-alarm states may occur in signal quality monitoring device 201 of FIG. 8 likewise in the signal quality monitoring device 200. In other words, since the producing and the canceling time period for each of BER-SD(xe2x88x926) alarm, BER-SF(xe2x88x923) alarm, LOF alarm and LOS alarm are defined as shown in table 1, the producing time period (787.5 ms) for a lower-grade BER-SD(xe2x88x926) alarm is longer than the canceling time period (725 ms) for a higher-grade BER-SF(xe2x88x923) alarm (i.e., sufficing Condition A). As a consequence, when a grade of low quality of a received signal shifts from higher-grade BER-SF(xe2x88x923) to lower grade BER-SD(xe2x88x926), a no-alarm state of 62.5 ms (787.5 msxe2x88x92725 ms) occurs during a time period corresponding to (i) (see reference numbers 301 and 302 in FIG. 10).
Similarly, since the producing time period (7.25 ms) for lower-grade BER-SF(xe2x88x923) alarm is longer than the canceling time period (250 xcexcs) for higher-grade LOF alarm (i.e., sufficing Condition A), a no-alarm state of 7 ms (7.25 msxe2x88x920.25 ms) occurs during time period corresponding to (i) in FIG. 9 (see reference numbers 303 and 304 in FIG. 10) when a grade of low quality of the received signal shifts from higher-grade LOF to lower-grade BER-SF(xe2x88x923).
Further, since the producing time period (3 ms) for lower-grade LOF alarm is longer than the canceling time period (100 xcexcs) for higher-grade LOS alarm (i.e., sufficing Condition A), a no-alarm state of 2.9 ms (3 msxe2x88x920.1 ms) occurs during time period (i) in FIG. 9 (see reference numbers 305 and 306 in FIG. 10) when a grade of low quality of the received signal shifts from higher-grade LOS to lower-grade LOF.
Each of the above three combinations of two different grades does not suffice Condition B (that is, the producing time period (7.25 ms/3 ms/12.5 xcexcs) for the higher-grade alarm is shorter than the canceling time period (78.75 ms/725 ms/250 xcexcs) for the lower-grade alarm). As a consequence, when a grade of low quality of a received signal shifts from a lower grade to a higher grade, continuous outputs of the corresponding higher-grade alarm is produced before continuous output of the corresponding lower-grade alarm is canceled (i.e., the continuous output of the lower-grade alarm is canceled by the producing of continuous output of the higher-grade alarm) whereupon no-alarm states does not occur at the time period corresponding to (ii) in FIG. 9.
Because APS triggers are created at the time when a grade of low quality of a received signal shifts (producing/canceling continuous output of an alarm) the occurrence of no-alarm states creates unnecessary APS triggers (see dotted-line Arrows 307 in FIGS. 9 and 10).
Issued alarms change one after another in a short time due to frequently shifting of a grade of low quality provides APS sections 113, 114, and 115 with a great number of APS triggers. Whereupon, when a CPU executes software to perform switching operations, the load on the CPU may prohibitively increase. Exceeding of the capacity of performance of the CPU may cause freezes on APS sections 113, 114, and 115.
With the foregoing problems in view, it is an object of the present invention to provide a signal transmitter and a signal quality monitoring device in which a no-alarm state, which has a possibility to occur in accordance with the lengths of a producing time period and a canceling time period for each of alarms of different grades, is prevented from occurring even when a grade of low quality of a received signal shifts from one to another grades.
To attain the above-mentioned object, according to the first generic feature of the present invention, there is provided a signal quality monitoring device for monitoring quality of a signal received by a transmitter and, when the signal is of low quality, selectively issuing one of two alarms with different grades in accordance with a grade of low quality of the signal, comprising: (1) a first-grade alarm producing/canceling section for successively discriminating whether the current signal is in a first grade of low quality and, when the current signal has stayed in the first grade of low quality for a first producing time period, producing continuous output of a first-grade alarm while, when the result of the discriminating becomes negative and the current signal has stayed off the first grade of low quality for a first canceling time period, canceling the continuous output of the first-grade alarm; (2) a second-grade alarm producing/canceling section for successively discriminating whether the current signal is in a second grade of low quality and, when the current signal has stayed in the second grade of low quality for a second producing time period, producing continuous output of a second-grade alarm while, when the result of the discriminating becomes negative and the current signal has stayed off the second grade of low quality for a second canceling time period, canceling the continuous output of the second-grade alarm; and (3) an alarm holding section for, when a grade of low quality of the current signal shifts from one to the other of the first and the second grades and the corresponding one of the first-grade alarm producing/canceling section and the second-grade alarm producing/canceling section has canceled the continuous output of the corresponding alarm as a consequence of expiration of the corresponding canceling time period, if the other of the first-grade alarm producing/canceling section and the second-grade alarm producing/canceling section discriminates that the current signal is in the other grade of low quality, holding the continuous output of the one alarm regardless of the cancellation by the one alarm producing/canceling section, until the other alarm producing/canceling section produces the continuous output of the corresponding other alarm as a consequence of expiration of the corresponding producing time period.
The signal monitoring device, when a grade of low quality of the current signal shifts from one to the other of the first and the second grades and the corresponding one of the first-grade alarm producing/canceling section and the second-grade alarm producing/canceling section has canceled the continuous output of the corresponding alarm as a consequence of expiration of the corresponding canceling time period, if the other of the first-grade alarm producing/canceling section and the second-grade alarm producing/canceling section discriminates that the current signal is in the other grade of low quality, holds the continuous output of the one alarm regardless of the cancellation by the one alarm producing/canceling section, until the other alarm producing/canceling section produces the continuous output of the corresponding other alarm as a consequence of expiration of the corresponding producing time period. As a result, it is possible to prevent the occurrence of a no-alarm state, in which no alarm is output despite the received signal being in either one of the first and the second grades of low quality and which is caused by the lengths of the producing time period and the canceling time period set for each of the first- and the second-grade alarms.
As a preferable feature, the alarm holding section may include: a first alarm holding circuit for, when a grade of low quality of the current signal shifts from the first grade to the second grade and the first-grade alarm producing/canceling section has canceled the continuous output of the first-grade alarm as a consequence of expiration of the first canceling time period, if the second-grade alarm producing/canceling section discriminates that the current signal is in the second grade of low quality, holding the continuous output of the first-grade alarm regardless of the cancellation by the first-grade alarm producing/canceling section, until the second-grade alarm producing/canceling section produces the continuous output of a second-grade alarm as a consequence of expiration the second producing time period; and a second alarm holding circuit for, when a grade of low quality of the current signal shifts from the second grade to the first grade and the second-grade alarm producing/canceling section has canceled the continuous output of the second-grade alarm as a consequence of expiration of the second canceling time period, if the first-grade alarm producing/canceling section discriminates that the current signal is in the first grade of low quality, holding the continuous output of the second-grade alarm regardless of the cancellation by the second-grade alarm producing/canceling section, until the first-grade alarm producing canceling section produces the continuous output of a first grade alarm as a consequence of expiration of the first producing time period.
With the first and the second alarm holding circuits, if the first producing time period is shorter than the second canceling time period, or if the first canceling period is longer than the second producing time period, it is possible to prevent a no-alarm state from occurring as a grade of low quality of the received signal shifts between the first and the second grades of low quality.
As a second generic feature, there is provided a signal transmitter communicably connected to a ring network, comprising: a receiving signal selector for selecting one from two signals, which are identical in contents and which are received by the signal transmitter from two directions of the ring network, based on signal quality information of each of the two signals; a signal quality monitoring section for monitoring quality of the two signals and, when each of the two signals is of low quality, selectively issuing one of two alarms with different grades, as the signal quality information, in accordance with a grade of low quality of the each of the two signals, the signal quality monitoring section, which is identical in construction with the signal quality monitoring device as the first generic feature.
The signal transmitter communicably connected to a ring network, which transmitter that selects one from two signals, which are identical in contents and which are received by the signal transmitter from two directions of the ring network, based on signal quality information of each of the two signals can prevent a no-alarm state from occurring whereupon the receiving signal selector does not have to perform unnecessary selecting operation caused by the occurrence of a no-alarm state.
As a third generic feature, a signal quality monitoring device for monitoring quality of a signal received by a transmitter and, when the signal is of low quality, selectively issuing one of a plurality of alarms with different grades in accordance with a grade of low quality of the signal, comprising: a plurality of producing/canceling sections, associated with the plurality of alarms with different grades respectively, each for successively discriminating whether the current signal is in the corresponding grade of low quality and, when the current signal has stayed in the corresponding grade of low quality for a producing time period, producing continuous output of an alarm of the corresponding grade while, when the result of the discriminating becomes negative and the current signal has stayed off the corresponding grade of low quality for a canceling time period, canceling the continuous output of the alarm of the corresponding grade; an alarm holding section for, when a grade of low quality of the current signal shifts from one to another of the plural alarms with different grades and the corresponding one of the plural alarm producing/canceling sections has canceled the continuous output of the corresponding one of the plural alarms of the plural grades as a consequence of expiration of the corresponding canceling time period, if the corresponding another of the plural alarm producing/canceling sections except the corresponding one alarm producing/canceling section discriminates that the current signal is in the corresponding another grade, holding the continuous output of the one alarm regardless of the cancellation by the corresponding one alarm producing/canceling section, until the another alarm producing/canceling section produces the continuous output of the corresponding another alarm as a consequence of expiration of the corresponding producing time period.
Since such a signal quality monitoring device, which selectively issues one of a plurality of alarms with different grades in accordance with a grade of low quality of the signal, it is possible to avoid a no-alarm state while a grade of low quality of the received signal shifts among the different grades by setting lengths of producing time periods and canceling time periods for alarms corresponding to the different grades of low quality to proper length.
As a fourth generic feature, a signal transmitter communicably connected to a ring network, comprising: a receiving signal selector for selecting one from two signals, which are identical in contents and which are received by the signal transmitter from two directions of the ring network, based on signal quality information of at least one of the two signals; a signal quality monitoring section, provided for the at least one signal, for monitoring quality of the at least one signal and, when the at least one signal is of low quality, selectively issuing one of a plurality of alarms with a plurality of different grades respectively, as the signal quality information, in accordance with a grade of low quality of the at least one signal, which signal quality monitoring section is identical in construction with the signal quality monitoring device of the third generic feature.
Also the last-named signal transmitter communicably connected to a ring network, which transmitter that selects one from two signals, which are identical in contents and which are received by the signal transmitter from two directions of the ring network, based on signal quality information of at least one of the two signals can prevent a no-alarm state from occurring whereupon the receiving signal selector does not have to perform unnecessary selecting operation caused by the occurrence of a no-alarm state.
Since the present invention has the above-mentioned generic and preferable features, it is possible to guarantee the following effective results and advantages:
(1) When a grade of low quality of the current signal shifts from one to the other of the first and the second grades and the corresponding one of the first-grade alarm producing/canceling section and the second-grade alarm producing/canceling section has canceled the continuous output of the corresponding alarm as a consequence of expiration of the corresponding canceling time period, if the other of the first-grade alarm producing/canceling section and the second-grade alarm producing/canceling section discriminates that the current signal is in the other grade of low quality, the continuous output of the one alarm is held regardless of the cancellation by the one alarm producing/canceling section, until the producing time period for the other alarm has been completed. As a result, it is possible to avoid a no-alarm state thereby generating no unnecessary alarms and avoiding unnecessary operations in apparatuses that are activated by the alarms.
(2) Since each alarm holding circuit is in the form of a combination of logical OR circuits and inverters, it is possible to prevent a no-alarm state from occurring by a simple configuration of the circuit.
(3) When the total number of outputs of alarms during a predetermined time period is equal to or greater than a predetermined threshold value whereupon a grade of low quality of the received signal frequently shifts for a short time period, continuous output of either one of the two or the plural alarms is held unless the total number of outputs of alarms during a subsequent predetermined time period becomes smaller than the predetermined threshold value irrespective of a state of shifting of grades of low quality. Therefore, it is further possible to generate no unnecessary alarms and to avoid unnecessary operations in apparatuses that are activated by the alarms.
(4) A signal transmitter, communicably connected to a ring network, selectively receives one from two signals, which are identical in contents and which are received by the signal transmitter from two directions of the ring network, based on signal quality information of each of the two signals whereupon a no-alarm state can be avoided. Further, it is possible to reduce unnecessary data selecting operation thereby minimizing the loads thereon.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.